Diagnostic imaging and therapeutic radiopharmaceuticals play an important role in modern medicine. Many of the important radionuclides used in current applications are metals positioned in the lanthanide series.1 This family of metals possesses diversity in nuclear properties that can be harnessed for both diagnostic and therapeutic applications. In nearly all cases, these metal ions are inherently toxic in a simple salt form and must be sequestered into an organic chelating compound (ligand) in order to render them biologically compatible. Furthermore, the ligand architecture is vitally important for creating a linking group for attachment to a biological targeting molecule.
Chelating agents for the lanthanide metal ions have been the subject of intense fundamental and applied research for many years driven in part by advancements in medicine.2 For example, the emergence of magnetic resonance imaging (MRI) as a new diagnostic modality brought with it the need for paramagnetic metal based contrast agents to enhance image quality, for this application gadolinium from the lanthanide series is preferred.3 As a result, there has been an exponential acceleration in the design and synthesis of new ligand systems that can hold up to the rigors of in vivo applications for MRI.4 Equally important is the fact that these same ligand systems are being recruited for other members of the lanthanide series (153Sm, 177Lu, 166Ho, 90Y) which possess highly desirable nuclear properties making them useful in radiopharmaceutical agents.5 The adaptability of similar ligands for all lanthanide ions is due to the very uniform and predictable properties intrinsic to the lanthanide series.
The critical prerequisites of all chelates intended for human use is that they remain stable in the body (no dissociation of the metal) and that they can be prepared reasonably fast. This latter point is more applicable to nuclear applications where isotope half-life is a critical consideration in the formulation process. Chelate stability is assessed in terms of thermodynamic and kinetic inertness. The most desirable chelates for biomedical applications are those that have the highest thermodynamic stability. However, these ligand systems usually require longer reaction times and additional energy input is needed to form the final complex.
One of the most popular ligands for both MRI and nuclear medicine has been diethylenetriaminepentaacetic acid (DTPA). DTPA is a linear ethyleneamine based chelating agent that forms thermodynamically stable complexes with the lanthanide series and displays reasonably fast kinetics of complexation. More recently the use of macrocyclic chelating agents based on 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) have become increasingly important in medicine due to the improved thermodynamic stability with lanthanides relative to DTPA.6 Nevertheless, both DTPA and DOTA have been modified for covalent attachment to biological targeting vectors and these bifunctional chelating agents (BFCA's) are now a cornerstone of the growing biotargeted radiopharmaceutical market.7 The perceived advantage of using a DTPA-based bifunctional chelating agent is that the kinetics of complexation is faster than for DOTA and this can be a significant consideration in view of the very low number of BFCA's present on a monoclonal antibody and the necessarily dilute complexation environment. However, an increasing body of knowledge suggests that low levels of metal do indeed dissociate in vivo from DTPA targeted conjugates which can be a serious consideration. Conversely, this type of toxicity issue is circumvented by employing a DOTA-based BFCA however the slow kinetics of complexation remains an issue to be addressed.
It would be advantageous to develop a bifunctional chelating agent (BFCA) that combined the rapid complexation kinetics of DTPA and the superior thermodynamic in vivo stability displayed by DOTA.8 A novel BFC possessing these desirable features would find broad utility in the radiopharmaceutical industry and furnish an unmet need in any application where fast complexation and long term stability is a requirement.
Numerous tetraazamacrocyclic ligand systems have been reported in the literature and shown to possess similar complexation properties as observed for related DOTA-type ligands. For example, U.S. Pat. Nos. 6,670,456 and 5,403,572 disclose generic molecules having a polyazabicyclic core and a linking group having a terminal functional group capable of forming a bond with a biomolecule connected to the backbone of the polyazabicyclic core, and an optional functionalised cyclic aliphatic or aromatic group connected through one of the nitrogen atoms of the polyazabicyclic core, which is also capable of forming a bond with the biomolecule. In these reports DOTA-type ligand systems are documented as being the gold standard for biological applications which require high thermodynamic and kinetic stability. Up to this point, examples of ligand systems capable of surpassing the performance of DOTA has been lacking.